LITANI RIVER, Lebanon — They came with shovels and a battered old tractor, armed with a bloody-minded determination to reopen the sole surviving lifeline connecting the south of Lebanon to the outside world.

It had looked grim just an hour earlier when Israeli missiles destroyed the makeshift bridge over the Litani River. Already unable to reach the area along Lebanon’s main southern highway, which was bombed in the early stages of the war, aid agencies warned of a humanitarian disaster in the making.

But then a minibus filled with Lebanese soldiers bounced along the rutted track that wound its way through an orange grove to the river. Often with just their bare hands, they shifted rocks and topsoil into the Litani’s green waters and within hours a new bridge had begun to appear.

It was a small but morale-boosting victory in a war in which the Lebanese army has played no role other than victim.

A number of servicemen have been killed in Israeli attacks on army outposts in south Beirut and elsewhere, and as they worked, the soldiers anxiously scanned the skies every time the roar of an Israeli drone broke the silence.

“Of course, we are scared,” Ghassan said, as he shifted rocks into the water. “But at least we are doing what we can. It may not be much, but it is still something.”

His anxiety was well founded.

Earlier in the day, a woman and her daughter were killed in an Israeli airstrike near an army checkpoint between the villages of Harouf and Dweir, just a few miles away.

They were among almost 30 people killed in Israeli airstrikes Sunday.

Despite the success in once again being able to ford the river, aid agencies still fear that their operations are likely to be heavily disrupted in coming days.

The Israeli army is determined to create an impregnable buffer zone south of the Litani before it hands over the area to a U.N. peacekeeping force.

The Israeli government told relief agencies yesterday that it could no longer guarantee the security of their convoys, which have already been struck by Israeli missiles.

“They have told us that they can provide no security guarantee that our convoy will not be attacked, so if we move, it will be at our own risk and peril,” the director of operations for the charity Doctors Without Borders, Christopher Stokes, said

It is not just the situation in the south that concerns aid workers. Lebanon is rapidly running out of fuel because of an Israeli naval blockade. Two oil tankers remain stranded at sea, unable to dock.

As long lines of people wait outside gas stations for what limited rations are available, at least two hospitals have been forced to close, according to government officials.

The strain is particularly being felt in Sidon, Lebanon’s third largest city, just north of Tyre. Its mainly Sunni population of 100,000 has more than doubled in the past three weeks, as Shiites fleeing the south cram into schools or sleep rough in the streets.

“Not only is nothing getting through, what resources there are have to be shared out by a much bigger population than normal,” a volunteer with the French charity Action Against Hunger, Ola Taha, said.

More than 200 people, many of whose houses were destroyed in Israeli air raids, sit side by side in overcrowded rooms at Sidon’s Palestine Martyrs School.

Just as in other such makeshift refugee camps, scabies and lice are spreading rapidly, and many of the children are suffering from diarrhea. The ill lie on mattresses in the sun, quietly moaning in pain.

This weekend, Israeli leaflets were dropped on the city urging residents to leave before future airstrikes. Few have heeded the warning.

Shiite refugees say they are tired of running; many Sunnis, who insist no Hezbollah presence exists in the city, say they refuse to be forced from their homes.